Impact of the vertical mixing induced by low-level jets on boundary layer ozone concentration

After sunset, a stable boundary layer (SBL) develops close to the ground, while the upper region of the daytime mixed layer becomes the residual layer (RL). Mixing between the SBL and RL is often quite limited and the RL is thought to be a reservoir for daytime mixed-layer pollutants under such conditions. However, ozone (O3) profiles observed in Maryland, U.S. suggest that the RL is not always a reservoir of O3 in that region. Nocturnal low-level jets (LLJs) and/or other mechanisms are speculated to enhance vertical mixing between the SBL and RL, which influences the vertical O3redistribution. Nocturnal surface O3 maxima, an RL with reduced O3 levels, and a concurrent strong LLJ were observed in Maryland on the night of August 9–10, 2010. Surface O3 measurements in the region and three-dimensional air quality simulations suggest that horizontal advection cannot explain the nocturnal O3 maxima and concurrent decrease of O3 levels within the RL. A sensitivity study with a single column (1D) chemistry model was performed to investigate the role of LLJs in generating turbulent mixing within the nighttime boundary layer and to identify related impacts on O3 concentrations at night and on the following day. The strong shear associated with the LLJ enhanced turbulent mixing and weakened the decoupling of the RL and SBL substantially. Ozone was actively mixed down from the RL to the surface, causing secondary nocturnal surface O3 maxima. Near the surface, O3 was efficiently removed by chemical reactions and dry deposition, which resulted in lower O3 peak values on the next day.